We intend to study mechanisms by which gene expression is regulated at the level of transcription. As a model system we will employ bacteriophage T7. During infection this virus the late genes are transcribed by a phage-specified RNA polymerase that is relatively simple in structure. The late RNAs are synthesized in two temporal classes (class II and class III). In vivo the purified enzyme discriminates between the promoters for these classes of RNA as a function of ionic strength, temperature, and the presence of helix destabilizing agents such as dimethyl sulfoxide. Preliminary investigations into the structure (sequence) of the class II and class III promoters have led to models in which the structure of the promoter may be correlated with function. We will investigate and test these predictions by: 1) sequencing additional T7 promoters, 2) comparing the response of individual promoters to variations in conditions in vivo and in vitro, 3) constructing recombinant (hybrid) class II/class III promoters and examining their properties. In addition, the structures of promoters in a related virus, bacteriophage T3, will be determined and compared with those of T7.